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Subject: Re: Radio static/Defroster
human <firstname.lastname@example.org> complains:
>At least one other lister (Huw Powell) has the same problem I do:
static between the rear defroster and the radio.< (that would be
>The improved reception from the new antenna only made me realize that
whenever I turned on my rear defroster I get static in the radio.
>Another data point: it is signal strength dependent. I'll be on
Route 4 in Durham NH listening to Maine Public Radio or WGBH in
Boston, and turning on the defroster will trash the reception. If I
then switch to WUNH, whose antenna I am then right under, the static
almost disappears. It's not being masked by volume - even if they are
playing something quiet this is apparent. So, dear listers, if you
have any knowledge of RF arcanery, what's up?
In my 1988 90q, the radio connects to two antennas, the one embedded
in the front window and the one utilizing the embedded rear window
defroster wiring. This latter antenna is the only feed to the AM
circuitry. Supposedly, for FM reception, the radio selects between
front and back dynamically as one drives.
The rear antenna cable, the defroster grid, and the defroster power
connect at a module in the left D pillar. This module presumably
passes dc power to the window grid, and rf from the window to the
radio. It may also amplify, as it does draw power via a separate
wire. As this module appears to be difficult to repair, checking for
a defective filter may be very difficult. If this is your problem, it
may be detected by a part swap with a known good unit. You may have a
defective ground, either in your coax cable, or for the window grid.
Remove the right D pillar covering to check the ground. (On my car,
the pillar moldings pull off forward and then up.) If there is rf
noise being injected into the defroster, it may be possible to detect
it by hooking the radio to another antenna, and moving it into close
proximity to the rear window and trying to detect an increase in
noise. This has to be more noise than bringing the antenna into
proximity with the car body, because there is also an effect of
reducing the signal, which will increase the noise after a point.
The phenomenon with WUNH vs. WGBH is due to the nature of FM signal
processing. Ideally, FM signals are fully limited in the radio, so
amplitude doesn't matter. Additive noise is ignored; only zero
crossings matter. When the signal is weak enough, then the noise is
additive. If you put a 10 element Yagi antenna on your car, and
pointed it toward WGBH's antenna, near Rts 128 and 9 in Massachusetts
(USA), you would surely get full limiting. Durham, NH, USA is
probably in the fringe for car radio reception of WGBH. For a real
test, try WBOQ at 104.9.
*** ...Kirby (Kirby A. Smith) ***
*** email@example.com ***
*** [=] firstname.lastname@example.org ***
*** Opinions expressed herein are entirely those of the author. ***